1. Field of the Invention
The present invention relates to a magnetic sensor system for detecting a physical quantity associated with the relative positional relationship between a scale and a magnetic sensor.
2. Description of the Related Art
In recent years, magnetic sensor systems have been employed to detect a physical quantity associated with the rotational movement or linear movement of a moving object in a variety of applications. Typically, a magnetic sensor system includes a scale and a magnetic sensor, and the magnetic sensor is configured to generate a signal associated with the relative positional relationship between the scale and the magnetic sensor.
The scale of the magnetic sensor system for use with a rotationally moving object is, in general, a rotating body that moves in response to the movement of the moving object. The rotating body can be, for example, a multipole-magnetized magnet having a plurality of pairs of N and S poles alternately arranged in a circumferential direction, or a gear having teeth formed of a magnetic material. In this case, the magnetic sensor system detects, for example, the rotational position and/or the rotational speed of the rotating body as the physical quantity.
The scale of the magnetic sensor system for use with a linearly moving object is, for example, a linear scale having a plurality of pairs of N and S poles arranged alternately in a linear configuration. In such a case, one of the linear scale and the magnetic sensor moves in response to the movement of the moving object, and the magnetic sensor system detects the relative position and/or speed of the linear scale with respect to the magnetic sensor as the physical quantity.
The magnetic sensor system is used, for example, in automobiles in order to detect the rotational speed of the axle shaft, the angle of the crank, the cam angle and so on. The magnetic sensor system for use with automobiles, in particular, is required to include means for detecting an abnormal event in the magnetic sensor.
JP 2003-194598A discloses an abnormal-event detector including a first sensor and a second sensor whose output values responsive to a change in physical quantity have mutually opposite output characteristics. The abnormal-event detector determines that one of the first and second sensors is abnormal when a total output value obtained by adding the output values of the first and second sensors is not a constant value. In this abnormal-event detector, the first sensor includes a first detection circuit, and the second sensor includes a second detection circuit. The first detection circuit and the second detection circuit are disposed to be adjacent to each other on one plane. Each detection circuit includes a bridge circuit constituted of four resistors. Each resistor is formed by depositing a thin film of NiCo in a staggered configuration on a substrate, and can be said to be an anisotropic magnetoresistive element.
The abnormal-event detector disclosed in JP 2003-194598A is configured so that, on the precondition that the direction of a magnetic flux passing through the first detection circuit and the direction of a magnetic flux passing through the second detection circuit are the same, the respective output values of the first sensor and the second sensor have mutually opposite output characteristics. The four resistors (anisotropic magnetoresistive elements) included in each detection circuit require a relatively large footprint. This leads to a relatively large difference in position between the first detection circuit and the second detection circuit. This abnormal-event detector is suitable for a system in which magnetic fluxes pass in the same direction through a wide range so that the magnetic flux passing through the first detection circuit and the magnetic flux passing through the second detection circuit are in the same direction. However, it is practically difficult to apply this detector to the aforementioned magnetic sensor system including the scale and the magnetic sensor. The reason for this will be described below.
In the aforementioned magnetic sensor system, continuous changes in the relative positional relationship between the scale and the magnetic sensor cause periodic changes in the direction of a magnetic field at a certain point in the magnetic sensor. Herein, the amount of a change in the relative positional relationship between the scale and the magnetic sensor that changes the direction of the magnetic field at a certain point by one period is referred to as one pitch. The abnormal-event detector disclosed in JP 2003-194598A may be applied to the magnetic sensor system so that the magnetic sensor includes the first and second detection circuits disclosed in JP 2003-194598A. In such a case, the difference between the positions of the first and second detection circuits would be significantly greater compared with one pitch. This would cause a significant difference between the direction of the magnetic flux passing through the first detection circuit and the direction of the magnetic flux passing through the second detection circuit. As a result, the respective output values of the first sensor and the second sensor could not have mutually opposite output characteristics. This would in turn cause the total output value not to be a constant value even when neither of the first and second sensors has any abnormal event, thus making it impossible to determine the presence of an abnormal event in the first or second sensor.
To cope with this, first and second sensors of the same configuration can conceivably be disposed apart from each other by ½ pitch to allow the output values of the first and second sensors to have mutually opposite output characteristics so that the presence of an abnormal event in the first or second sensor can be determined from the total output value. In this case, however, there is a problem as follows. In the case of a magnetic sensor system, the scale may differ depending on the system to which the magnetic sensor system is applied, and the magnitude of one pitch can vary accordingly. Thus, even if the first and second sensors are disposed apart from each other by ½ pitch in accordance with a certain scale to construct a magnetic sensor system, a change in the magnitude of one pitch resulting from a change of the scale would cause the difference between the positions of the first and second sensors not to be ½ pitch any longer. This would in turn cause the total output value not to be a constant value even when neither of the first and second sensors has any abnormal event, thus making it impossible to determine the presence of an abnormal event in the first or second sensor.